Which of the following is the excretory organ in birds?

Birds are vertebrates, and like all vertebrates, their primary excretory organs are the kidneys. The kidneys filter nitrogenous waste from the blood and produce uric acid, which is the main nitrogenous waste product in birds. Uric acid is semi-solid and requires very little water for excretion, an important adaptation for flight since it reduces the weight of water that must be carried.

Birds lack a urinary bladder (in most species); instead, uric acid passes from the kidneys through the ureters directly into the cloaca, where it is expelled along with faeces as a white, paste-like substance.

Each of the other structures listed is an excretory organ of a different group of animals:

• Nephridia are the excretory structures of annelids such as earthworms.
• Malpighian tubules are the excretory organs of insects and arachnids.
• Green glands (antennal glands) are the excretory organs of crustaceans such as prawns and crayfish.

A useful revision tip: match each excretory organ to its animal group - kidneys (vertebrates), nephridia (annelids), Malpighian tubules (insects), green glands (crustaceans), flame cells (flatworms), and contractile vacuoles (protozoa).